The microstrip technique is a planar technique with applications to making signal transmission lines and to making antennas constituting a coupling between such lines and radiated waves. It employs conductive patches and/or strips formed on the top surface of a thin dielectric substrate which separates them from a conductive ground layer on the bottom surface of the substrate. A patch of the above kind is typically wider than a strip of the above kind and its shape and dimensions constitute important characteristics of the antenna. The substrate is typically in the form of a rectangular plane sheet of constant thickness. This is in no way obligatory, however. In particular, it is known that an exponential variation in the thickness of the substrate widens the bandwidth of an antenna of the above kind and that the shape of the sheet can depart from the rectangular shape. The electric field lines extend through the substrate between the strip or the patch and the ground layer. The above technique differs from various other techniques that also use conductive elements on a thin substrate, namely:
the stripline technique in which a strip is confined between the bottom ground layer and a top ground layer which in the case of an antenna must include a slot to enable coupling with the radiated waves, PA1 slotted line techniques in which the electric field is established between two parts of a conductive layer formed on the top surface of the substrate and separated from each other by a slot which in the case of an antenna must typically open into a wider opening facilitating coupling with the radiated waves, for example by forming a resonant structure, and PA1 the coplanar line technique in which the electric field is established on the top surface of the substrate and symmetrically between a central conductive strip and two conductive areas on respective opposite sides of the strip from which they are separated by respective slots. In the case of an antenna, the strip is typically connected to a wider patch to form a resonant structure providing a coupling with the radiated waves. PA1 the configuration of the patches, which can include slots, possibly radiating slots, PA1 the presence and the location of any short-circuits and of electrical models representative of short-circuits, although the latter cannot always be deemed to be equivalent, even approximately, to perfect short-circuits of zero impedance, and PA1 coupling devices included in such antennas for coupling their resonant structures to a signal processing unit such as a transmitter, and the location of such devices. PA1 a plane dielectric substrate; PA1 a conductor constituting a ground plane on the bottom surface of said substrate; PA1 three conductive areas on the top surface of the substrate each having an elongate shape imparting to the antenna a double-C or three-branch candlestick shape; PA1 an antenna coupling device common to all the conductive areas. PA1 it must be a multifrequency antenna, i.e. it must be able to transmit and/or to receive efficiently on more than one operating frequency, PA1 it must be possible to connect it to a single processing unit by means of a single connecting line for all operating frequencies, and PA1 to achieve this it must not be necessary to use a frequency multiplexer or demultiplexer. PA1 The antenna being a half-wave antenna, its longitudinal dimension can be a problem if it is required to miniaturize the antenna. PA1 The need to provide impedance converter means complicates its manufacture. PA1 It is difficult to adjust the resonant frequencies precisely to required values. PA1 The above drawback relating to it being a half-wave antenna. PA1 The polarizations of waves transmitted at the two resonant frequencies of the antenna are necessarily crossed polarizations, which can complicate the manufacture of certain telecommunication systems using the antenna. PA1 The above drawback relating to it being a half-wave antenna. PA1 The incorporation of localized short-circuits complicates the manufacture of the antenna. PA1 Likewise the feeding of the antenna via a coaxial line. PA1 to limit the dimensions of a multifrequency antenna, PA1 to enable easy and precise adjustment of the operating frequencies of the antenna, and PA1 to enable the use of a single coupling device whose impedance can easily be tuned to more than one operating frequency. PA1 a plane dielectric substrate; PA1 a conductor constituting a ground plane on the bottom surface of said substrate; PA1 a plurality of conductive zones on the top surface of the substrate and each having an elongate shape imparting a candlestick shape to the antenna; PA1 an antenna coupling device common to all the conductive zones; PA1 and wherein said conductive zones are separated from each other by slots the widths of which are very much less than the operating wavelengths of the antenna; PA1 wherein said conductive zones are sufficiently decoupled from each other to enable various resonances to occur, respectively, in various areas formed by said zones, said resonances being at least approximately of the quarter-wave type; PA1 and wherein each of said zones has an electric field node fixed by at least one short-circuit to the ground plane and said short-circuit is in the vicinity of the base of the candlestick.
With regard to the manufacture of antennas, the following description will on occasion and for simplicity be restricted to the case of a transmit antenna connected to a transmitter. It must nevertheless be understood that the arrangements described could equally apply to receive antennas connected to a receiver. With the same aim of simplicity it will be assumed that the substrate is in the form of a horizontal sheet.
Broadly speaking, a distinction can be made between two fundamental types of resonant structure that can be implemented in microstrip technology. The first type might be called a "half-wave" structure. The antenna is then a "half-wave" or "electric" antenna. Assuming that one dimension of the patch constitutes a length and extends in a longitudinal direction, the length is substantially equal to half the wavelength of an electromagnetic wave propagating in that direction in the line consisted by the ground plane, the substrate, and the patch. Coupling with the radiated waves occurs at the ends of the length, the ends being in regions where the amplitude of the electric field in the substrate is maximal.
A second type of resonant structure that can be implemented using the same technology might be called a "quarter-wave" structure. The antenna is then a "quarter-wave" or "magnetic" antenna. It differs from a half-wave antenna firstly in that its patch has a length substantially equal to one fourth of the wavelength, with the length of the patch and the wavelength being defined as above, and secondly in that there is a hard short-circuit at one end of the length between the ground plane and the patch so as to impose a quarter-wave type resonance with a node of the electric field fixed by the short-circuit. The coupling with the radiated waves occurs at the other end of the length, which is in the region in which the amplitude of the electric field through the substrate is maximal.
In practice various types of resonance can occur in such antennas. They depend in particular on:
For a given antenna configuration there may be more than one resonant mode enabling use of the antenna at a plurality of frequencies corresponding to the resonant modes.
An antenna of the above kind is typically coupled to a signal processing unit such as a transmitter not only by means of a coupling device included in the antenna but also by means of a connecting line external to the antenna and connecting the coupling device to The signal processing unit. Considering an overall functional system including the signal processing unit, the connecting line, the coupling device, and the resonant structure, the coupling device and the connecting line must be made so that the system has a uniform impedance throughout its length, which avoids spurious reflections opposing good coupling.
In the case of a transmit antenna having a resonant structure, the respective functions of the coupling device, of the connecting line, and of the antenna are as follows: the function of the connecting line is to transport a radio frequency or microwave frequency signal from the transmitter to the terminals of the antenna. All along a line of the above kind the signal propagates in the form of a traveling wave without any significant modification of its characteristics, at least in theory. The function of the coupling device is to convert the signal supplied by the connecting line to a form in which it can excite resonance of the antenna, i.e. the energy of the traveling wave carrying the signal must be transferred to a standing wave established in the antenna with characteristics defined by the antenna. As for the antenna, it transfers energy from the standing wave to a wave that is radiated into space. The signal supplied by the transmitter is therefore converted a first time from the form of a traveling wave to that of a standing wave and then a second time to the form of a radiated wave. In the case of a receive antenna the signal takes the same forms in the same units but the conversions are carried out in the opposite direction and in the reverse order.
The connecting lines can be implemented in a nonplanar technology, for example in the form of coaxial lines.
Planar technology antennas are used in various types of equipment. They include mobile telephones, base stations for mobile telephones, automobiles, aircraft, and missiles. In the case of a mobile telephone, the continuous nature of the bottom ground layer of the antenna means that the radiated power intercepted by the body of the user of the device is easily limited. In the case of automobiles, and above all in the case of an aircraft or a missile whose outside surface is a metal surface and has a curved profile to minimize drag, the antenna can be conformed to that profile so as not to generate any unwanted additional drag.
European patent application EP 0 749 176 describes a microstrip antenna including:
The area in the middle of the candlestick has an electric field node fixed by a series of short-circuits to the ground plane, the series of short-circuits being disposed all along the axis of symmetry of this area.
The conductive areas are separated from each other by relatively wide slots (0.7 cm wide for a wavelength of 3.3 cm) so that an antenna of this kind can be smaller than a prior art antenna for a given wavelength. However, the antenna is not able to operate properly at more than one frequency, for example in a multiband mobile telephone.
The present invention is more particularly concerned with the situation in which an antenna of the above kind must have the following properties:
Various prior art microstrip antennas have the above properties. They will now be discussed:
A first prior art antenna is described in patent document U.S. Pat. No. 4,766,440 (Gegan). The patch 10 of that antenna has a continuous curved U-shape slot entirely within the patch. The slot radiates and produces an additional resonance mode of the antenna. By an appropriate choice of its shape and its dimensions, it provides required values of the frequencies of the resonance modes, making it possible to associate two modes with crossed linear polarizations to transmit a circular polarization wave. The feed line terminates at a coupling device which is in the form of a microstrip line (see above) but which can also be regarded as a coplanar line because the microstrip is in the plane of the patch and penetrates between two notches in the patch. The device has impedance converter means for matching it to the various input impedances presented by the line at the various resonant frequencies used as operating frequencies.
This first prior art antenna has the following drawbacks:
A second prior art antenna is described in patent document U.S. Pat. No. 4,692,769 (Gegan). Its patch has a slot along a circular arc or a straight line segment inside the patch. The slot produces an additional resonance mode. The ends of the circular arc slot are enlarged to impart the same value to the antenna input impedance for the various operating frequencies. This second prior art antenna has the following drawbacks:
A third prior art antenna is described in patent document U.S. Pat. No. 4,771,291 (LO et al.). Its patch includes slots along respective straight line segments within the patch. These slots reduce the difference between the two operating frequencies. Localized short-circuits also reduce this difference. They are provided by conductors passing through the substrate.
The third prior art antenna has the following drawbacks: